This invention relates to a glow plug adapted to preheat the interior of a cylinder of a diesel engine.
Since the starting characteristic of a diesel engine at low temperatures is generally poor it is usual to install a glow plug in the cylinder so as to increase the temperature in the cylinder by passing electric current through the glow plug, thereby improving the starting characteristic of the engine.
As the glow tube, it has been used as a sheath heater in which a helical heating wire is embedded in a powder of heat resistant electric insulation packed in a tubular sheath made of heat resistant metal. Since heating with the sheath heater is an indirect heating, it takes a long time to raise the temperature in the cylinder. Thus, the heat generated by passing current through the heating wire is transmitted to the sheath through the heat insulating powder and then radiated into the cylinder, so that the efficiency of heat transmission is low. For example, it takes about ten seconds to increase the temperature in the cylinder to 800.degree. C. If the current is increased to shorten the preheating time, the heating wire would melt or the sheath would be broken due to high temperature. In the manufacture of a sheath heater it is necessary to take care not to cause short circuiting between the heating wire and the sheath at the time of packing the insulating powder. For this reason it is necessary to precoat the heating wire with an electric insulator or to form an electric insulating layer on the inner surface of the sheath.
In Japanese Preliminary Publication of patent specification No. 109536/1979 dated Aug. 28, 1979, a ceramic heater utilized as a glow plug is disclosed in which a multilayer substrate technique is used for improving the efficiency of heat transmission. According to this technique a plurality of thin disc shaped ceramic green sheets, each printed with a resistor, are laminated into an integral rod. With this construction, however, it is necessary to prepare a number of green sheets, each printed with a resistor, on a narrow surface and to connect in series or parallel both ends of respective resistors exposed on the periphery of the stacked sheets for electrically interconnecting resistors on respective sheets. This often fails to connect the resistors in a desired pattern.